1. Field of the Invention
This invention relates to a process of manufacturing high-strength sintered members having a hard-wearing surface, particularly for manufacturing such parts for use in the valve timing mechanisms of internal combustion engines, where at least that portion of said sintered member which is formed with said hard-wearing surface is formed in that a carbon-containing powder mixture, which comprises an iron alloy that contains at least one carbide-forming allowing element of the group VIa of the periodic system, is compacted to form a compact, which is then subjected to liquid-phase sintering.
2. Description of the Prior Art
In order to provide cams for use on a camshaft or other members of valve timing mechanisms of internal combustion engines, which cams or other members meet stringent requirements regarding their wear resistance and fatigue strength, it is known (EP-A-303 809) to make such members in that a powder mixture is compacted which comprises an iron alloy powder that contains carbide-forming alloying elements of the groups Vb and VIb of the periodic system, and graphite powder in the amount which is required for a formation of carbides. The compacts are then sintered at a temperature which slightly exceeds the solidus temperature and the compact which has thus been subjected to liquid-phase sintering is compacted to at least 99% of its theoretical density by isostatic hot pressing. A major disadvantage of that known process resides in that the isostatic hot pressing of the presintered compacts involves a considerable expenditure but such isostatic pressing is essential to ensure a uniform distribution of the carbides at the required density. Whereas sintering at a sintering temperature slightly above the solidus temperature permits a uniform distribution of carbides, this will be possible only with a comparatively high void ratio. Besides, it is not possible to use lower-melting alloying elements owing to the high temperatures which are required for hot pressing and such lower-melting elements would melt at the pressing temperatures and would then emerge through the still existing pores during the pressing operation.
It is finally known from DE-A-3 907 886) that the cams of a camshaft may be made to have a hard-wearing external layer and a cam body by a powder-metallurgical process, which comprises liquid-phase sintering and in which the law compacts, which differ in their shrinking behavior, are fitted onto a steel shaft, so that a strong bond is obtained between the hard-wearing layer and the cam body and between the cam body and the steel shaft when the sintering has been performed. In that case the hard-wearing external layer is constituted by an iron-carbon-nickel-chromium-molybdenum alloy. But that alloy will not withstand high loads because, for instance, nickel cannot form carbides, which would be essential for a high wear resistance, and nickel-containing materials tend to form austenite so that the fatigue strength is reduced.